To what degree does Cycles' sample level impact other passes?
Depends on the pass. Z and Object/Mat ID are not antialiased at all, and require only 1 sample. Normal, mist, and UV are also just data/stats, but since they are antialiased you need a few samples to get a smooth result. 8-16 is usually enough. Vector is a bit of a weird case that I confess I don't fully understand. It doesn't appear to be antialiased in the normal sense, but does seem to gather some multisample data - possibly time variance?

The remaining passes are just components of the main render, so they're essentially dependent on the samples rate of the full scene. However, since different components denoise at different rates, some passes may clear up more quickly than others. The "color" passes (including emit and environment) are just texture readouts and generally don't take more samples the normal/UV/mist. AO, shadow, and the direct/indirect passes contain the lighting portion of your render and generally one of them (usually one of the indirect ones) will be the last component to clear up.